How Many Senses Does a Shark Have?

We learn early on that humans have five senses: vision, hearing, smell,
touch, and taste. As we learn more about ourselves, we come to understand
that we can also sense such things as hunger, pain, and temperature. Most of
us also claim a 'sense' of humor and style, although we recognize that this
is not the same kind of sense as vision or hearing. As we learn more about
the world around us, we learn that many animals have astoundingly acute
senses, including several that we do not.

We humans are fascinated by these 'extra' senses. Inevitably, and despite
the fact that many of them are beyond our experience or understanding, we
want to know what these other senses detect and how well. Which brings us to
sharks. Sharks are most famous for their phenomenal sense of smell. But,
depending upon which book or authority one consults, sharks may have as many
as 13 sensory systems - eight more than we are accustomed to. But do they
really?

All sensory systems depend on receptor cells that respond to energy or
chemicals in the environment by changing their electric charge. This
electric change, in turn, induces a series of events that signal the brain,
where the environmental energy or chemical is interpreted. In practice,
there are only a few fundamental ways of accomplishing this neat trick.

Consider the senses of smell and taste. These are usually regarded as
separate experiences: we smell with our nostrils and taste with our tongues.
Yet both these senses function in precisely the same way: a chemical sample
is dissolved in a watery fluid so that it fits into a receptor cell like a
key inside a lock. The tightness of this chemical fit dictates the nature of
the electrical signal sent to the brain. Since smell and taste are based on
the same mechanism, they can be thought of as different versions of the same
basic sense, chemoreception (chemo = chemical, reception = to receive).

If we were to group all of our senses - no matter how different they may
seem - by fundamental mechanism, we discover that we do not really have five
senses at all. We have only three: photoreception (vision), chemoreception
(smell and taste), and mechanoreception (touch and hearing). Similarly,
sharks do not have 13 senses, they have four: the three we have plus
electroreception.

Electroreception is found in many marine and freshwater fishes, several
amphibians, as well as a few mammals. These electrosensitive mammals include
the 'primitive' egg-laying monotremes - the Platypus (Ornithorhynchus
anatinus), the Short-Beaked (Tachyglossus aculeatus) and
Long-Beaked (Zaglossus bruijni) Echidnas - and at least one 'higher'
placental mammal, the Star-Nosed Mole (Condylura cristata). All
vertebrates share a common ancestor. Since the ability to sense electric
fields has re-appeared several times in discrete vertebrate lineages,
electroreception must be a very ancient ability.

Thus, at some time in our long-forgotten past, our distant mammalian
ancestors were probably sensitive to weak electric fields. This mysterious
voice continues to speak clearly to sharks and a mere handful of other
creatures, but is now lost to us. It is difficult to not feel a little
short-changed. But we can take some pride in that, of all the species with
whom we share this planet, we are the only ones who can sense the loss of
something we have never known.